The 28-year-old plowed through the Angels, dueling Matt Shoemaker through four scoreless innings. In the fifth, O'Sullivan finally received the run support he was looking for from a struggling Red Sox offense.
Boston scored seven times during an RBI double-filled fifth in which every batter in the lineup reached base.
"I knew it was going to be important to put up some zeros early, to give our offense a chance to get on the board first. After that all I could do was stay within myself and try not to put too much pressure on it. Not try and do too much," O'Sullivan said.
O'Sullivan lasted five-plus innings, surrendering four hits, two runs and three walks. His outing was just the second time in 12 games a Red Sox starter allowed two or fewer runs.
It wasn't until the sixth when he gave up back-to-back base hits that O'Sullivan exited after 76 pitches. The starter wanted to go a little farther than he did but couldn't believe the crowd's standing ovation as he walked to the dugout.
"It was only five [innings] coming out of the sixth, but I'd say that was one of the highlights of my career for sure," O'Sullivan said.
Farrell praised the Boston faithful for understanding how important O'Sullivan's start was to the team.
"I think it speaks to how intelligent our fans are," Farrell said. "They know that Sean comes up here in a spot-start situation. A guy that's been up once before, has paid his dues, has shown a lot of perseverance over the course of his career. When he walks off the mound, not allowing a run to that point, our fans understand who he is, where he's from and the job he did today."
O'Sullivan bounced around before landing in Boston, pitching for the Royals, Phillies and Padres. But the first club to claim him was the Angels as a third-round Draft pick in 2005. The San Diego native debuted for the Angels in 2009, but now hopes to make a home in Boston.
"It's been a lot of ups and downs," O'Sullivan said. "One of the guys said, "Hey, you're facing your former team." I was like, 'Well, half the league's my former team.' Pretty good shot I've worn the other jersey. I think I've enjoyed my time here more than anywhere else at this point. The team's great, the staff's great, the environment, the fans are awesome. If I could make this place home, that'd be fantastic."
Deesha Thosar is reporter for MLB.com based in Boston.